Jonathan,
No worries, I think we are zeroing in on the key points now. Though I
don't yet know how ocean chemists will feel about the end results!
But I appreciate the thorough analysis brought to the issue.
In your option (1) (sea_water_pH universally), it isn't clear to me
what other attribute would indicate the scale difference. I had
understood that if the standard name and units were the same, CF users
would assume the data could be exchanged, from a numerical standpoint
anyway. If that is true, or if the specific attribute where this scale
information must be entered, and the controlled vocabulary to use, is
not specified, option (1) does not seem attractive. The likely outcome
would be the attribute is unspecified or ambiguous, rendering the
associated values useless to many potential users of the data.
In case (2) (different geophysical quantities), we may be a little
stuck. I quote from my own ocean chemist: "as an ocean chemist, I know
EXACTLY what is meant by sea_water_pH_total_scale, and so do all the
other ocean chemists." He believes other terminology will be opaque
and ambiguous, and therefore essentially useless. As I understand the
situation, the entire system of related chemistry in sea water is
considered different depending on which scale is in use, so it is a
pervasive concept in this domain, not simply an obscure name for this
one measurement.
Since I am unable to personally verify this last assertion with all
those other ocean chemists, and you might not want to take my word for
it anyway, it may be that you would like to verify this perspective
with other experts in the domain? (I can provide some names offline if
you would like to do that.) I think the reason to consider doing so
in this case is that if there is indeed some ambiguity in the name you
choose, then it may apply equally well in the future to a different
scale, and this would be problematic.
If, after such a discussion, or regardless of that point, you choose a
name that seems more appropriate to you, we can always create our
preferred name in another vocabulary and map it to yours. (Eventually
I am sure the CF convention will support this technique in an embedded
way. :->) So all is not lost, so long as the definition makes clear
that the name you choose refers precisely to sea water pH total
scale. Such an approach seems sufficiently usable to me, and we can
bear the brunt of name translation for our cadre of ocean chemists.
John
On May 2, 2009, at 5:53 AM, Jonathan Gregory wrote:
> Dear John
>
> Thanks for your thoughtful email. I follow the argument but in the
> end I do
> not (yet) agree with the conclusion, that sea_water_pH_total_scale
> should be a
> standard name. I am sorry if you find that exasperating. As in
> previous such
> discussions, it is not because I am suggesting that you or your pH
> expert are
> mistaken about what terms are normally used and what they mean. My
> difficulty
> is that "total scale" is not informative to a general scientific
> readership,
> which CF standard names are designed to be.
>
> I've had a look at Zeebe and Wolf-Gladrow. Thanks for that reference.
> Wikipedia summarises similar ideas. I also looked at a IUPAC
> document on the
> subject. It seems that problem of definition arises because pH is
> actually
> impossible to measure, as defined! Hence the true value is somewhat
> unknowable.
> These different scales represent different methods of measurement and
> estimation. I think we could take two views about that:
>
> * The different pH scales are actually trying to measure the same
> geophysical
> quantity. In that case the standard name should just be
> sea_water_pH. Then the
> scale is an aspect of the method of measurement, and should be
> recorded in
> some other attribute. This is somewhat related to the other thread,
> about
> quantities relating to the techniques of measurement.
>
> * The different pH scales are different geophysical quantities. In
> that case
> they need distinct standard names, and it ought to be possible to
> indicate
> in some non-jargon way what the difference refers to, even though it
> couldn't
> be fully described in the name, so we still need a definition.
> Following this
> route, the total scale could be called
> sea_water_pH_including_bisulfate_ions,
> for instance. (A side-issue is a question of whether to call them
> bisulfate
> or the IUPAC name of hydrogen_sulfate.)
>
> pH is a dimensionless quantity. It has no units. It's the logarithm of
> a number which does have units, so the units of that quantity need
> to be
> specified in the definition.
>
>> Yet the second
>> sentence of the following statement does not hold true for pH, given
>> the depths at which these measurements are being made:
>>
>> On Apr 30, 2009, at 9:22 AM, Jonathan Gregory wrote:
>>> Boussinesq models (most ocean climate models are Boussinesq) treat
>>> density
>>> as constant 1000 kg m-3 except in the computation of pressure
>>> gradients, where
>>> it matters to the dynamics. Therefore in dealing with concentrations
>>> of
>>> tracers, per kg and per litre are identical and to choose one or the
>>> other
>>> would be arbitrary and hence unhelpful for data exchange.
>
> Actually this is a digression now, since all the quantities
> concerned are
> logs of mol per kg. However, I maintain the above is a correct
> statement about
> ocean *models*. I did not say it's a correct statement about nature!
> It is
> not, as you point out. However, Boussinesq models are only
> approximate. In
> such models, that distinction could not be made. No-one has yet
> requested
> pH for an ocean model, but I expect it will come up sometime.
>
> Best wishes
>
> Jonathan
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--------
John Graybeal <mailto:graybeal at mbari.org> -- 831-775-1956
Monterey Bay Aquarium Research Institute
Marine Metadata Interoperability Project:
http://marinemetadata.org
Received on Mon May 04 2009 - 17:47:43 BST
